Evaluation of soya bean and sesbania sesban for soil fertility replenishment, increased rice yield and enhanced incomes

Abstract

This study focused on the development of practical agro-ecosystem measures aimed at enhancing agricultural productivity while simultaneously reducing malaria vector populations for improved household incomes in irrigated rice farming. The main strategy entailed testing alternation of rice with legumes. Soya bean (Glycine max L.JMarr) and Sesbania sesban were tested as the preceding crops against the conventional rice-fallow and the rice-rice cultivation systems. Among the preceding crops, rice produced the highest amount of biomass followed by Soya bean varieties EAI 3600 and Duiker. However, Soya bean residue had a higher nutrient content than rice straw and fallow treatment (weeds).

The fallow-rice cropping system gave the lowest preceding plants (weeds) biomass quantities followed by Sesbania sesban. Soya bean cultivation allowed easier working of soil for the succeeding rice crop, on the normally hard and sticky, black cotton soil. Soya bean cultivation before rice resulted in the highest soil Nand P nutrient replenishment while Potassium was highest in the fallow (weeds) treatment.

The Soya bean cultivation before rice saved 33% of in-organic fertilizer N. After cultivation of the subsequent rice crop, soil nutrients N, P, K and Zn content were low in all plots thus emphasizing the need to alternate rice with a legume crop to replenish soil fertility. Organic carbon was moderately available in all plots. Significant mean grain yield differences (p<0.05) among the test soil fertility replenishment preceding crops were obtained. Rice produced mean grain yield of 4946kg ha-1 while Soya bean gave an average of2215kg ha' grain yield. Among the subsequent rice treatments, yields from Soya bean-rice cropping system were generally the highest (6307kg ha'), Rice-rice cropping system gave the lowest yield (4438kg ha') followed by rice-fallow system (4516kg ha').

There were significant (p<0.05) yield differences between rice-Soya bean (4111kg ha') and rice-rice (3167kg ha') treatments at zero N Kg ha' level. Rice-rice treatment recorded the lowest subsequent rice grain yield at both the cropping systems and the different levels of N (3167,4633,5100 and 6]67 kg ha-1 rice grain yield at 0, 40, 80, and 120 kg N ha' respectively Soya bean-rice treatments generally gave higher subsequent rice grain yield at all N levels (4111,5343,5197, 6829kg ha-1 yield at 0, 40, 80, and 120 kg N ha-1 respectively). The superior performance of soya bean-rice system is attributed to averagely higher plant residue nutrient content and the increased nodulation observed in Soya beans treatments in paddy soil that could have enhanced Nitrogen-fixation resulting in improved soil fertility and hence higher yield. Malaria vector larval population densities for Anopheline species L 1 L2, (151 instars) in

Soya bean, fallow and rice treatment were 0, 2 and 4, respectively and 0, 1 and 4, respectively for L3L4, (2nd instars). Hence, Soya bean-rice system resulting in modification of paddy field ecosystem leading to significant elimination of the malaria vector breeding sites unlike the rice-rice and rice-fallow systems. The reduction of mosquito larva densities would hence contribute positively to the fight against malaria disease impact. The Soya bean-rice system also resulted in production of the high protein rich soya grain that would be suitable for improvement of household nutrition and income. Adoption of Soya bean-rice cropping system would therefore cause ecosystem modification that would eventually lead to improved soil fertility, reduced malaria disease incidences and improve farmers' health. This would in turn result in allocation of uninterrupted timefor agricultural productivity and hence increase rice output and income.

Partial budget, Gross margin analysis and benefit cost ratio analysis indicated multiple benefits that are associated with the Soya bean-rice cropping system. The benefits includes 16% rice grain yield increase, 1730kgha-1 Soya bean grain yield production and Kshs8381 ha' in inorganic N fertilizer saving. The Soya bean-rice cropping system resulted in a net income increase ofKshs 61,317. The gross margins for the preceding rice crop and Soya bean were Kshs 27,945 ha' and Kshs 47,011 ha' respectively.

The benefit-cost ratio CBCR) at a price of Kshs 30 ,per kg of rice and Soya bean were 1.23 and 2.95 respectively. Yields were 4946Kg ha' and 2371Kg ha' for rice and Soya beans respectively. There is therefore, need to encourage farmers to rotate rice with soya bean for increased food production and ;income. The Soya bean cropping system should be introduced as an alternative to rice monoculture in Mwea Irrigation Scheme for increased rice production, improved incomes, improved human health, and preservation of the environment.